Metal base cookware induction heating apparatus having improved power supply and gating control circuit using infra-red temperature sensor and improved induction heating coil arrangement

ABSTRACT

A metal based cookware induction heating apparatus having an improved power supply and control circuit comprised of a chopper inverter including a gate controlled power thyristor coupled to and supplying an induction heating coil with periodic energization currents. A full wave rectifier supplies rectified, unfiltered, unidirectional high voltage excitation potential to the chopper inverter which converts the excitation potential to a comparatively high frequency of the order of 20 kilohertz for supply to the induction heating coil. A gating circuit is provided for driving the chopper inverter directly from the rectified high voltage excitation potential supplied by the rectifier.

United States Patent 1 1 Peters, Jr.

[54] METAL BASE COOKWARE INDUCTION HEATING APPARATUS HAVING IMPROVED POWER SUPPLY AND GATING CONTROL CIRCUIT USING INFRA-RED TEMPERATURE SENSOR AND IMPROVED INDUCTION HEATING COIL ARRANGEMENT Jan. 9, 1973 3,553,567 1/1971 Pesce et a1 ..32l/4 3,594,630 7/1971 swam ..321/45 c 3,349,314 10/1967 Giannamore ..321/45 C FOREIGN PATENTS OR APPLlCATlONS 1,157,711 7/1969 Great Britain 219/1049 OTHER PUBLICATlONS [75] mentor: Philip Greenwich W. C. Fry, An Ultrasonic Power Source Utilizing A Solid State Switching Device, IRE lnternatl Conven- [73] Assignee: Environment/One Corporation, Record, 6, 1PP- Schenectady, NY. Primary Examiner-J. V. Truhe [22] Aim] 1971 Assistant ExaminerB. A. Reynolds [21] Appl. No.: 131,648 Attorney-Charles W. Helzer 57 ABSTRACT [52] 11.8. C1. ..2l9/10.49, 219/10.77, 219/1079, 1

219/502, 321/4, 321/10, 321/12, 321/45 C A metal based cookware induction heating apparatus [51] Int. Cl. ..H05b 5/04 h g n improv d power supply and control circuit [58] Field of Search..219/ 10.49, 10.67, 10.75, 10.79, comprised of a chopper inverter including a gate con- 219/l0.77, 502; 321/4, 12, 27, 45 C; trolled power thyristor coupled to and supplying an in- 323/DIG. 1 duction heating coil with periodic energization currents. A full wave rectifier supplies rectified, unfil- [56] References Cited tered, unidirectional high voltage excitation potential to the chopper inverter which converts the excitation UNITED STATES PATENTS potential to a comparatively high frequency of the 3,129,366 4/1964 order of 20 kilohertz for supply to the induction heat- 3 475, 74 I0 19 9 ing coil. A gating circuit is provided for driving the 3,035,143 5/1962 Leatherman ..2l9/ 10.77 chopper inverter directly from the rectified high volt- 3,332,036 7/1967 K pp nh gen et all9/l0-75 X age excitation potential supplied by the rectifier. 3,275,784 9/1966 Merrett 3,448,367 6/1969 Corey ..321/12 44 Claims, 41 Drawing Figures l4 l5 l5 R .QQQQ f II E 32 19 2 c f as N I I T GATING v n INVERTER I PULSE 33 Z COMMUTATING I AMPLIFIER A S I AND N lB cIRcuIT AND I2 i TRIGGER PULSE E5 2 E R SUPPLY GENEFATOR 1 9| GATING COIL-LOAD rv PULSE 23 22 TRANS FORMER 0c SUPPLY START UP "zERo" POINT FOR SENSOR DELAY swITcIIING 3| I aAMPLlFlER INHIBIT CONTROL L I 1 I I I a 2 J 1G HEATING /25 RATE I CONTROL TENPERATIIRE l SENSOR AND I32 24/ AMPLIFIER PATENTED JAN 9 I973 sum near 16 FIG 2B FIG 2A FIG: 2

PATENTEDJAM 9197s SHEET DQUF 16 'IIIIIII/l VIIIIIIIA PATENTEDJAN 9|975' SHEET UBUF 16 PATENIEDJ/III 9|973 3.710.062 sum 08m 16 I |2I I30 201 '22 g FIG. 4B

LN I35 I20 INVERSE GAIN WITH DIAL SET DIRECT GAIN WITH 200 DIAL SET 0 I00 EAR LSET I L2 15 A-)- I v i I7 l8 Ci g E v; C; SCR 0 M prrrm V \IVUV PATENTEDJAN 9 I973 SHEET lOUF 16 PATENTED JAN 9 I975 SHEET lUUF16 PATENTEUJAN 9:975 3.710.062

SHEET lSUF 1s FIG. 13A

EUTRAL l5 (20H 2 I LINE H TO NEUTRAL F/G. 13B

:3 V L2 c 3 0 W o ale TEMP. SENSOR TEMP- SENSOR 3 SUPPLY AMPLIFIER FIG? 14 PATENTED JAN 9 I975 3. 71 O O62 SHEET 16 OF 16 VACUUM SYSTEM FIG 15 DISTANCE l G I F/6T16A SENSOR METAL BASE COOKWARE INDUCTION HEATING APPARATUS HAVING IMPROVED POWER SUPPLY AND GATING CONTROL CIRCUIT USING lNFlRA-RED TEMPERATURE SENSOR AND IMPROVED INDUCTION HEATING COIL ARRANGEMENT The gating circuit includes an additional low voltage, direct current excitation potential connected to supply to a timing circuit charging capacitor. A voltage responsive switch such as a SUS or PUT or their equivalents is rendered conductive upon the timing circuit reaching a preset value and in turn renders conductive a constant current high voltage switching transistor connected in series with the primary winding of a pulse transformer across the output of the additional low voltage derived from the main full wave rectifier. The pulse transformer in turn has its secondary winding connected to the control gate of the power thyristor included in the chopper inverter.

The chopper inverter consists of a series filter inductor having an inductance L a bidirectional switching device composed of a conducting gate-controlled thyristor switching SCR and feedback diode. The SCR and diode are connected in series circuit relationship with the filter inductor across the supply terminals to the chopper inverter with the filter inductor being interposed between the thyristor switching device and the supply terminals. A commutating inductor having an inductance L, and a commutating capacitor having a capacitance C, are connected in series circuit relationship across the thyristor switching device and are tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t, during each cycle of operation. Output power from the chopper inverter is derived from across one of the commutating components, and a gating circuit is coupled to the control gate of the thyristor switching device of the type recited above for rendering the thyristor device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period t in each cycle of operation where T t t such that the value w t equals substantially 1r/2 radians and where 1/ V L C whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is substantially independent ofload.

The gating circuit controlling operation of the chopper inverter in turn is controlled by a temperature sensing arrangement that includes an infra-red heat sensor device arranged to view the metal-based cookware or other object whose temperature is to be controlled. A constant current amplifier is connected in series circuit relationship with the infra-red heat sensor device across a source of low voltage direct current excitation potential, and means are provided for periodically interrupting the view of the infrared heat sensor device to produce an output alternating current signal component whose amplitude is indicative of the temperature of the metal-based cookware or other object being heated. An alternating current amplifier including an adjustable temperature set resistor is coupled to the output from the infra-red heat sensor device with the setting of the adjustable temperature set resistor serving to control the gain and establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude. An output on-off control amplifier either in the form of a Schmitt trigger type of circuit or reed relay is coupled to and controlled by the output from the alternating current amplifier for producing an amplified output onoff control signal representative of the temperature of the metal-based cookware or other object whose temperature is being controlled in accordance with the temperature setting of the adjustable temperature set resistor.

The on-off output from the temperature sensing arrangement controls an inhibit circuit connected to and controlling a zero point sensing and control circuit that in turn controls the operation of a soft starting zero point switching device connected intermediate the rectified ac supply terminals and the thyristor switching device employed in the chopper inverter whereby both radio frequency interference effects and audible noise production by the inductive heating coil during energization periods from the chopper inverter is substantially reduced due to the fact that energization of the coil under the control of the temperature sensing arrangement is inhibited until only the beginning or near the beginning of each half cycle of the rectified full wave potential appearing at the output from the high voltage rectifier. To further reduce RFl effects, a smoothing inductor having an inductance L and a smoothing capacitor having a capacitance C is connected in series circuit relationship across at least one of the commutating components of the chopper inverter power supply circuit. The combined impedance of the commutating capacitor, the smoothing inductor L and the smoothing capacitor C is capacitive in nature and is series resonant with the commutating inductor L to establish the commutating period r,. The smoothing inductor and the capacitor serve to shape the output current flowing through the smoothing inductor which is used as the inductive heating coil to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects. Additionally, a capacitor switching arrangement is provided for the commutating capacitor C, which is formed from a plurality of parallel connected capacitors and power rate controlling switching means for switching selected ones of the parallel connected capacitors into effective circuit relationship in the chopper inverter circuit in accordance with a desired rate of power production.

Where the improved induction heating apparatus is employed in connection with metal-based cookware, the inductive heating coil preferably is in the form of a helically shaped planar inductive heating coil and is disposed under a heat transparent member such as an infra red transmissive pyroceramic that can form a smooth continuous tabletop over the induction heating unit and itself does not become heated except by conduction and convection from the metal-based cookware. The inductive heating coil excites and heats the metal-based cookware directly so that all heat is generated only by the metal-based cookware and no excess waste heat is produced in the manner of known resistance heating units. Since the temperature sensing units directly views the bottom of the metal-based cookware through the heat transparent member and controls the application of power to the induction heating coil, temperature control is directly related to the actual temperature of the pan or other object being heated by the induction heating apparatus. As a consequence, a considerable improvement in efficiency and reduction of waste heat results. Temperature control is accurate, rapid in response and there is no overshoot in temperature such as is characteristic of temperature control systems which control the temperature of a heat source which is external to the object being heated. Improved arrangements for the heat transparent member, infra-red heat detector, chopper device, and cooling fan together with preferred designs for the inductive heating coil with ferrite core or without to maximize coupling to the metal-base cookware or other object being heated, are described whereby further improvements in efficiency of operation, economy of components and minimum size are achieved.

BACKGROUND OF INVENTION 1. Field of Invention This invention relates primarily to a new and improved induction heating apparatus designed for use with metal-based cookware.

More particularly, the invention relates to a new and improved induction heating apparatus employing novel chopper inverter power supply circuits having improved operating characteristics and novel control gating circuit arrangements using new and improved infrared heat sensing and coil arrangements for directly sensing and controlling the temperature of metal-based cookware or other objects being heated with an induction heating coil. The improved induction heating apparatus provides rapid, safe, clean and efficient heating which can be accurately controlled in a manner which minimizes the production and loss of waste heat, and accomplishes this with compact equipment of relatively low cost and not too complex design.

2. Prior Art Situation Heretofore electrically operated home cooking appliances have employed resistance heating almost universally to convert electric into heat energy for home cooking purposes. While there are exceptions, such as are exemplified by the micro-wave heating ovens, for the most part, known electrical heating appliances use resistance heating. With such appliances the heat generated is proportional to PR where I is the current supplied to the resistance heating element and R is the resistance of the heating element. By switching or otherwise controlling the values of R and/or controlling I, the heat generated by a resistance heating unit can be controlled comparatively easily dependant upon the degree of control desired. However, with all resistance heating units the heat generated is controlled by controlling in some manner the electrical energy supplied to the unit. By prior calibration, the amount of heat generated by the resistance heating unit, and hence the resulting temperature of the object being heated are somewhat and only roughly related.

Consider, for example, the well known helically coiled, flat top electric stove heating units of the Calrod type. These units are controlled by calibrated switches on the stove which at a given setting produce a measurable quantity of I R generated heat that then is thermally coupled to the object being heated such as a pan. A great deal of the generated heat is wasted and contributes to thermal pollution of the environment in which the stove is located. If the stove is located in an air conditioned kitchen, not only does the waste heat contribute to inefficiency, but also the additional cooling load imposed on the air conditioner by the waste heat contributes to further inefficiency. Over a considerable operating period such as the summer months of June through August, the accumulated inefficiencies can contribute to a significant wasteage in power and increased utility bills not to mention the added discomfort in the kitchen during the cooking periods and increased loading on the utility system during periods when predictably power shortages are most likely to occur.

In addition to the inefficiencies mentioned above, presently available electric 1 R cooking units provide no reliable control over the temperature of the pan or other objects being heated. This lack of reliable control is due to the differences in the transfer of heat to pans fabricated from different metals such as aluminum, copper, iron or alloys of metal, and the difficulty of relating the efficiency of thermal coupling to the several different types of cookware and calibrated settings of the control knob for adjusting heat generation by the resistance heating units. Hence, at best the calibrated settings at the control knobs are only an educated guess at what is the actual temperature of the pan or other object being heated. To overcome the above discussed and other deficiencies inherent in electrical resistance heating apparatus, the present invention was devised.

SUMMARY OF INVENTION It is therefore a primary object of the invention to provide a new and improved induction heating apparatus for use with metal-base cookware and other similar objects to be heated, which is compact, relatively low cost and simple in design and provides safe, clean and highly efficient heating.

Another object of the invention is to provide an induction heating apparatus of the above type which is capable of directly sensing and controlling the temperature of the cookware or other object being heated.

A further object of the invention is to provide an improved chopper-inverter power supply circuit for use primarily in induction heating equipment and which is reliable and safe in operation over a wide range of loadmg.

Still another object of the invention is the provision of a novel gating control circuit for inverter power supply circuits which is capable of providing reliable gating-on pulses of sufficient strength and time duration to assure complete turn-on and control over the operation oflarge power rated inverters.

A still further object of the invention is the provision of improved induction heating coil designs and temperature sensing arrangements for use in induction heating apparatus having the characterisitics noted above.

In practicing the invention, an improved power supply and control circuit arrangement for induction heating apparatus is provided and includes an improved chopper-inverter power supply circuit under the control of a gate controlled power thyristor such as an SCR that is coupled to and supplies an induction heating load with periodic energization currents. Full wave rectifier means are provided for supplying a rectified, unfiltered direct current high voltage excitation potential to the chopper inverter circuit which is gated on periodically by an improved gating circuit means. One improved gating circuit means includes a timing circuit supplied from the voltage across the SCR which operates a voltage responsive switch such as a SUS or PUT that in turn gates on momentarily a constant current, a high voltage switching transistor connected in series circuit relationship with the primary winding of a pulse transformer across a filtered dc voltage supply obtained from the high voltage rectifier. The switching transistor amplifies the input from the timing circuit; and the amplified signal appearing at the secondary terminals of the pulse transformer is fed to the control gate of the large power rated thyristor comprising a part ofthe chopper inverter power supply circuit.

The improved chopper-inverter power supply circuit includes power supply terminal means for connection to a source of electric energy for energizing the chopper inverter. A filter inductor having an inductance L and a bidirectional conducting gate controlled semiconductor thyristor such as an SCR and a parallel diode are connected in series circuit relationship across the power supply terminal means with the filter inductor being interposed between the thyristor switching device and the supply terminal means. A commutating inductor having an inductance L and a commutating capacitor having a capacitance C are connected in series circuit relatioship across the thyristor switching device and are tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t during each cycle of operation. Means for deriving output power are coupled across at least one of the commutating components and gating circuit means are coupled to the control gate of the thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period in each cycle of operation where T= t, +t such that the value 1 equals substantially 17/2 radians and where w l/ VL C, whereby the reapplied forward voltage across the semiconductor switching device following each conduction interval is substantially independent of load. For improved operation, a smoothing inductor L and a smoothing capacitor C are connected in series circuit relationship across the commutating capacitor of the chopper inverter circuit and serve to shape the output current supplied by the chopper inverter to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects. With such an arrangement, the smoothing inductor may comprise an inductive heating coil, and the load is coupled to and derives output power from the circuit across the smoothing inductor. Further, a greater level of power is coupled to the load for given levels of peak voltage, and current on the SCR than is available when power is taken inductively from the commutating inductor.

To further reduce radio frequency interference emission effects (RF!) a soft starting zero point energization switching means may be connected in the circuit intermediate the output from the rectifier and the chopper inverter power supply circuit. Zero point sensing and turn-on circuit means are coupled to and control the soft start zero point switching means for switching the zero point switching means on only at or near the beginning of each half cycle of the rectified unfiltered unidirectional excitation potential appearing at the output of the high voltage rectifier means. Inhibit control means for inhibiting operation of the zero point sensing and turn-on circuit means are provided and are controlled by a temperature sensing means for sensing the temperature of metal-based cookware or other objects being heated by the inductive heating coil. The temperature sensing means derives on-off control signals in response to the temperature dropping below or raising above a preset level and supplies the on-off control signals to the inhibit control means for thereby controlling output power and hence heat generated in the metal base cookware or other objects by the inductive heating coil.

An improved temperature set and control circuit is provided which includes an infra-red heat sensor device arranged to view the bottom of the metal based cookware or other objects being heated directly. The infra-red heat sensor is connected in series circuit relationship with a constant current source across a source of low voltage direct current excitation potential and means are provided for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal component whose amplitude is indicative of the temperature of the metal based cookware or other objects being heated. An alternating current amplifier is supplied from the output of the infra-red heat sensor device and includes an adjustable temperature set impedance for controlling the gain of the alternating current amplifier whereby the setting of the temperature set impedance serves to establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude. The output from the alternating current amplifier then in turn controls an output on-off control amplifier in the form of either a Schmitt type of trigger or a reed relay which in turn is coupled to and controls the inhibit control means of the soft start zero point switching device which controls energization of the chopper inverter gating circuit means and the chopper inverter power supply circuit.

The temperature sensing and control arrangement preferably comprises a planar helically shaped inductive coil having a central opening disposed under an insulating heat transparent supporting member or other similar arrangement such as an insulating support member having a heat transparent window upon which the metal-based cookware is supported. The infra-red heat detecting device is supported within a suitable heat reflector and views directly the bottom of the metal-base cookware through the cental opening of the induction heating coil and the heat transparent supporting member for directly controlling the temperature of the cooking vessel. A light chopping device is disposed intermediate the heat transparent supporting member and the infra-red heat detector device for periodically interrupting the view of the infra-red heat detector to thereby produce the alternating current signal component and also preferably includes fan 

1. An improved induction heating unit power supply and control circuit comprising an induction heating coil, inverter circuit means including a gate controlled power thyristor coupled to and supplying the induction heating coil with periodic energization currents, rectifier means for supplying a rectified unfiltered unidirectional high voltage excitation potential for said inverter circuit means and gating circuit means for periodically gating on said inverter circuit means, said gating circuit means including means for deriving a low voltage unidirectional self excitation potential from the high voltage rectified unfiltered unidirectional excitation potential supplied from said rectifier means, timing circuit charging means supplied by said low voltage excitation potential, voltage responsive switching means connected to and controlled by said timing circuit charging means, said voltage responsive switching means being rendered conductive upon the voltage of said timing circuit charging means attaining a preset value, a semiconductor switching device having a relatively flat current vs voltage conducting characteristic and having its load terminals connected to the output from the rectifier means, the control gate of said constant current semiconductor switching device being connected to and supplied by the voltage responsive switching means, and gating signal output circuit means connected in series circuit relationship with the constant current high voltage switching device and adapted for connection to the control gate of the gate controlled power thyristor comprising a part of the inverter circuit means.
 2. An improved induction heating unit power supply circuit and control according to claim 1 further including soft-starting zero point energization switching means connected intermediate the output of the rectifier means and the inverter circuit means, and zero point sensing and turn-on control circuit means coupled to and controlling said soft starting zero point energization switching means for rendering the same conductive only near the beginning of a half cycle of the rectified unfiltered unidirectional excitation potential.
 3. An improved induction heating unit power supply circuit and control according to claim 1 further including inhibit control means for inhibiting operation of said zero point sensing and turn-on circuit means, and load condition sensing means for sensing the operating condition of a load supplied by the inverter circuit means and deriving an on-off control signal in response to the load condition for controlling operation of said inhibit control means.
 4. An improved induction heating unit power supply circuit and control according to claim 1 wherein the inverter circuit means is a chopper inverter comprised by a filter inductor having an inductance L2, a bidirectional conducting gate control semiconductor thyristor switching device connected in series circuit relationship with the filter inductor across the output from said rectifier means with the filter inductor interposed between the thyristor switching device and the rectifier means, commutating inductor means having an inductance L1 and commutating capacitor means having a capacitance C1 connected in series circuit relationship across the thyristor switching device and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, said commutating capacitor and inductor means including as a part thereof the induction heating coil, the gating circuit means being coupled to the control gate of the thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operation period T for the chopper inverter including a quiescent charging period t2 in each cycle of operation where T t1 + t2 such that the value omega 2t2 equals substantially pi /2 radians or greater and where omega 2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is maintained substantially independent of load.
 5. An improved induction heating unit power supply circuit and control according to claim 1 wherein the inverter circuit means comprises a chopper inverter circuit that produces periodic pulsed currents having radio frequency interference emission effects and the circuit further includes a smoothing inductor and a smoothing capacitor connected in circuit relationship across the chopper inverter circuit which serve to shape the output current supplied by the chopper inverter circuit to the load to substantially a sinusoidal waveshape having little or no radio frequency interference effects, and wherein the smoothing inductor comprises the induction heating coil.
 6. An improved induction heating coil power supply circuit and control according to claim 1 further including soft starting zero point energization switching means connected between the rectifier means and the chopper inverter circuit means, zero point sensing and turn-on circuit means coupled to said soft start zero point switching means for switching said means-on only at or near the beginning of each half cycle of the rectified unfiltered unidirectional excitation potential, inhibit control means for inhibiting operation of said zero point sensing and turn-on circuit means, and temperature sensing means for sensing the temperature of an object being heated by the inductive heating coil and deriving an on-off control signal in response to the temperature rising above a preset level for controlling operation of said inhibit control means.
 7. An improved induction cooking unit power supply circuit and control according to claim 4 further including a smoothing inductor having an inductance L3 and smoothing capacitor having a capacitance C3 connected in series circuit relationship across at least one of the commutating components and having impedance values such that the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor is capacitive in nature and series resonates with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects, and the load is coupled to and derives output power from the smoothing inductor, and wherein the smoothing inductor comprises the induction heating coil.
 8. An improved induction cooking unit power supply circuit and control according to claim 7 further including soft starting zeRo point energization switching means connected between the output of the rectifier means and the chopper inverter circuit, zero point sensing and turn-on circuit means coupled to the soft starting zero point switching means for switching said last mentioned switching means-on- only at or near the beginning of each half cycle of the rectified unfiltered unidirectional excitation potential, inhibit control means for inhibiting operation of said zero point sensing and turn-on circuit means, and temperature sensing and control means for sensing the temperature of an object being heated by said induction heating coil and deriving an on-off control signal in response to the temperature of the object for controlling operating of said inhibit control means.
 9. An improved induction heating unit power supply circuit and control according to claim 8 wherein said temperature sensing and control means comprises an infra-red heat sensor device arranged to view the object being heated by said inductive heating coil, a constant current amplifier connected in series circuit relationship with the infra-red heat sensor device across a source of low voltage direct current excitation potential, means for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal whose amplitude is indicative of the temperature of the object being heated, an alternating current amplifier, an adjustable temperature set impedance coupled to and controlling the gain of said alternating current amplifier, the setting of said adjustable temperature set impedance serving to establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude, on-off control amplifier means coupled to and controlled by the output from said alternating current amplifier for producing an amplified on-off output signal representative of the temperature of the object being heated by said inductive heating coil relative to the temperature setting of the adjustable temperature set impedance, said on-off output control signal being supplied to the inhibit control means and means for supplying operating bias potentials from a common reference point on the source of low voltage excitation potential supplying the alternating current amplifier and the on-off control amplifier means whereby the effect of variations in the excitation potential is minimized.
 10. An improved induction heating unit power supply circuit and control according to claim 9 wherein the infra-red heat sensor device comprises a lead sulfide semiconductor cell, the constant current amplifier comprises a field effect transistor having a feedback resistor connected in series with its drain-source electrodes and having a feedback connection from the feedback resistor back to the gate of the field effect transistor for maintaining constant current through the amplifier and the series connected lead sulfide cell, the alternating current amplifier comprises an integrated circuit operational amplifier having inverting and non-inverting input terminals and an output terminal with the output signal from the infra-red heat sensor device being AC coupled to the non-inverting input terminal, and the adjustable temperature set impedance comprises three variable resistors connected in series circuit relationship between the output terminal and the inverting input terminal with the outer most variable resistors serving as minimum and maximum temperature limit settings and the middle variable resistor serving to establish the desired operating temperature setting, and means for supplying the inverting and non-inverting input terminals with operating bias potentials from a common point on the source of low voltage excitation potential supplying the alternating current amplifier and infra-red sensor device, and the output on-off control and amplifier means comprises on-off bistable Schmitt trigger circuit means responsive to the output from said alternating current ampLifier means for producing an output electric potential having a first value under conditions where the temperature sensed by the lead sulfide cell is below the temperature level established by the setting of the adjustable temperature set impedance, and for producing an output potential of a second different value under conditions where the sensed temperature is above the temperature setting of the adjustable temperature set impedance, the output from said Schmitt trigger circuit means being coupled to and controlling operation of said inhibit control means.
 11. An improved induction heating unit power supply circuit according to claim 10 wherein the inductive heating coil comprises a planar helically shaped inductive heating coil, a pyroceramic insulating heat transparent support for supporting metal base cooking vessels capable of being inductively heated, the helically shaped inductive heating coil having a central opening therein and being disposed under the pyroceramic heat transparent support, the lead sulfide infra-red heat detecting cell being disposed beneath the central opening so as to view the bottom of a metal base cooking vessel supported on the pyroceramic heat transparent insulating supporting member, the lead sulfide cell being supported within a heat reflector for concentrating infra-red heat rays from the bottom of the cooking vessel onto the lead sulfide cell, and an infra-red light chopper interposed in the infra-red light path between the bottom of the cooking vessel through the pyroceramic heat transparent pan supporting member and the central opening and the lead sulfide cell for periodically interrupting the view of the lead sulfide cell.
 12. An improved induction heating unit power supply circuit and control according to claim 11 wherein the infra-red light chopper also includes fan means for producing a flow of cooling air through the ambient atmosphere surrounding the control and wherein the planar helically shaped inductive heating coil comprises a twisted multi-conductor insulated cable helically wound in a flat plane and having inner and outer terminal points, said multi-conductor cable being fabricated from multiple strands of insulated conductors interwoven together to form the insulated cable with the outer terminal points of certain respective strands being connected back to the inner terminal points of adjacent strands to provide a desired amount of inductive coupling to the metal base cooking vessel heated by the coil, and means for connecting the inner and outer terminal points of at least one of the multiple strands to the chopper inverter circuit as the smoothing inductor, and further including a pan-shaped ferrite core member having a central opening supporting the inductive heating coil below the surface of the heat transparent supporting member for concentrating the lines of magnetic flux up into the inductively heated cooking vessel.
 13. An induction heating unit chopper inverter power supply circuit including in combination supply terminal means adapted to be connected to a source of electric energy for energizing the chopper inverter, a filter inductor having an inductance L2, a bidirectional conducting gate controlled semiconductor thyristor switching device connected in series circuit relationship with the filter inductor across said supply terminal means with the filter inductor interposed between the thyristor switching device and the supply terminal means, commutating inductor means having an inductance L1 and commutating capacitor means having a capacitance C1 connected in series circuit relationship across the thyristor switching device and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, said commutating inductor and capacitor means including as a part thereof an induction heating coil that serves as a load, and gating circuit means coupled to the control gAte of said thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period t2 in each cycle of operation where T t1 + t2 such that the value omega 2t2 equals substantially pi /2 radians or greater and where omega 2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is substantially independent of load.
 14. An induction heating unit chopper inverter power supply circuit according to claim 13 further including a filter capacitor having a capacitance C2 connected across the series connected filter inductor and thyristor switching device.
 15. An induction heating unit chopper inverter power supply circuit according to claim 13 further including rectifier means connected across said supply terminal means intermediate a source of alternating current supply and said thyristor switching device for supplying rectified unfiltered unidirectional excitation potentials to said thyristor switching device across said supply terminal means from said alternating current supply.
 16. An induction heating unit chopper inverter power supply circuit according to claim 15 wherein the gate controlled semiconductor thyristor switching device is unidirectional conducting and the rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means and wherein the filter inductor is located between the supply of alternating current potential and the rectifier means.
 17. An induction heating unit chopper inverter power supply circuit according to claim 13 wherein a single commutating inductor is provided which also comprises the induction heating coil.
 18. An induction heating unit chopper inverter power supply circuit according to claim 13 further including on-off control circuit means for rendering said gating circuit means effective to control turn-on of the thyristor switching device in accordance with the power demands on the circuit.
 19. An induction heating unit chopper inverter power circuit according to claim 18 further including temperature sensing and control means for sensing the temperature of an object being heated and controlling said on-off control circuit means in response to the sensed temperature, and wherein said on-off control circuit means includes soft starting zero point energization switching means coupled to the supply terminal means and controlling energization of said gating circuit means, zero point sensing turn-on and control circuit means coupled to said soft start zero point switching means for switching said soft start switching means on only at the beginning of a half cycle of the excitation potential appearing across the supply terminal means, inhibit control means for inhibiting operation of said zero point sensing and control circuit means, said temperature sensing means controlling operation of said inhibit control means, and a smoothing inductor having an inductance L3 and a smoothing capacitor having a capacitance C3 connected in series circuit relationship across at least one of said commutating components, the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor being capacitive in nature and series resonant with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects and wherein the smoothing inductor comprises the induction heating coil.
 20. An induction heating unit chopper inverter power supply circuit according to claim 13 further including soft starting zero point energization switching means coupled to the supply terminal means and controlling energization of said gating circuit means, and zero point sensing and turn-on control circuit means coupled to said soft start zero point switching means for switching said soft start switching means on only at the beginning of a half cycle of the excitation potential appearing across the supply terminal means.
 21. An induction heating unit chopper inverter power supply circuit according to claim 13 further including a smoothing inductor having an impedance L3 and a smoothing capacitor having a capacitance C3 connected in series circuit relationship across at least on of said commutating components, the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor being capacitive in nature and series resonant with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects, the load being coupled to and deriving output power from across the smoothing inductor, and the smoothing inductor comprising an inductive heating coil.
 22. An induction heating unit chopper inverter power supply circuit according to claim 13 wherein said commutating capacitor comprises a plurality of parallel connected capacitors and power rate controlling switching means for switching selected ones of said parallel connected capacitors into effective circuit relationship in accordance with a desired rate of power production.
 23. An induction heating unit chopper inverter power supply circuit according to claim 21 further including rectifier means connected across said supply terminal means intermediate a source of alternating current supply and said thyristor switching device for supplying rectified unfiltered unidirection excitation potentials to said thyristor switching device across said supply terminal means from said alternating current supply.
 24. An induction heating unit chopper inverter power supply circuit according to claim 23 further including on-off control circuit means for rendering said gating circuit means effective to control turn-on of the thyristor switching device in accordance with the power demands on the circuit.
 25. An induction heating unit chopper inverter power supply circuit according to claim 24 further including soft starting zero point energization switching means coupled to the supply terminal means and controlling energization of said gating circuit means, and zero point sensing and turn-on control circuit means coupled to said soft start zero point switching means for switching said soft start switching means on only at the beginning of a half cycle of the excitation potential appearing across the supply terminal means.
 26. An induction heating unit chopper inverter power supply circuit according to claim 25 wherein the gate controlled semiconductor thyristor switching device is unidirectional conducting and the rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means and wherein the filter inductor is located between the supply of alternating current potential and the rectifier means.
 27. An induction heating unit chopper inverter power supply circuit according to claim 23 wherein the gate controlled semiconductor thyristor switching device is unidirectional conducting and the rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating Current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means and wherein the filter inductor is located between the supply of alternating current potential and the rectifier means.
 28. An induction heating unit chopper inverter power supply circuit according to claim 27 further including a filter capacitor having a capacitance C2 connected across the series connected filter inductor and thyristor switching device.
 29. An induction heating unit power supply according to claim 21 further including selectively operable inhibit control means coupled to said gating circuit means for selectively inhibiting operation of said gating circuit means, and delay means for introducing a delay period intermediate selective operation of the inhibit means in a direction to remove the inhibit and enable operation of the gating circuit means after initial start-up.
 30. An induction heating unit power supply according to claim 13 further including selectively operable inhibit control means coupled to said gating circuit means for selectively inhibiting operation of said gating circuit means, and delay means for introducing a delay period intermediate selective operation of the inhibit means in a direction to remove the inhibit and enable operation of the gating circuit means after initial start-up.
 31. A temperature sensing and control signal deriving arrangement for an inductively heated cooking vessel comprising a heat transparent supporting member for a cooking vessel, an infra-red heat detecting device viewing the cooking vessel directly through the heat transparent supporting member for deriving an output electric control signal that is directly related to the temperature of the cooking vessel, and a planar, spirally wound inductive heating coil having a central opening disposed under the insulating heat transparent supporting member for use with cooking vessels that are metal based and capable of being inductively heated, the infra-red heat detecting device viewing the bottom of the cooking vessel through the central opening in the heating coil.
 32. A temperature sensing and control signal deriving arrangement according to claim 31 further including an infra-red light chopping device disposed intermediate the heat transparent supporting member and the infra-red heat detecting device to thereby produce an alternating current signal component in the output of the infra-red heat sensor device whose amplitude is indicative of the temperature of the cooking vessel.
 33. A temperature sensing and control signal deriving arrangement according to claim 32 wherein the light chopping device also includes fan means for producing a flow of cooling medium through the ambient atmosphere surrounding the arrangement.
 34. A temperature set and control signal deriving arrangement according to claim 31 further including a pan-shaped ferrite core member having a central opening supporting the inductive heating coil below the surface of the heat transparent supporting member for concentrating the lines of magnetic flux up into the inductively heated cooking vessel.
 35. A temperature sensing and control signal deriving arrangement according to claim 31 further including a heat reflector with the infra-red heat detecting device being supported within the heat reflector for concentrating the infra-red heat rays emitted from the cooking vessel upon the infra-red heat detecting device.
 36. A temperature sensing and control signal deriving arrangement according to claim 31 wherein the heat transparent insulating supporting member is pyroceramic, the spirally-wound planar inductive heating coil is disposed under the pyroceramic heat transparent member within pan-shaped ferrite core member with both the heating coil and ferrite core member having central apertures formed therein for allowing the infra-red heat detecting device to directly view the bottom of a cooking vessel supported on the pyroceramic heat transparent member, the infra-red heat detecting device comprises a lead sulfide semiconductor cell and is disposed within a heat reflector for concentrating infra-red rays emitted from the cooking vessel onto the lead sulfide cell, and a light chopper is interposed between the lead sulfide semiconductor cell and the light path from the bottom of the heated cooking vessel for periodically interrupting the view of the lead sulfide infra-red heat detecting cell and includes fan means for circulating cooling air through the arrangement.
 37. A temperature sensing and control signal deriving arrangement according to claim 36 wherein the spirally wound planar inductive coil comprises a twisted multi-conductor insulated cable helically wound in a flat plane defining a central opening and having inner and outer terminal points, said multi-conductor cable being fabricated from multiple strands of insulated conductors interwoven together to form the insulated cable with the outer terminal points of the certain respective strands being connected back to the inner terminal points of adjacent strands to provide a desired amount of inductive coupling to an object to be heated by the coil, and means for connecting a source of electric energy across an inner and outer terminal point of at least one of the multiple strands.
 38. An improved induction heating unit power supply and control circuit comprising an induction heating coil, inverter circuit means including a gate controlled power thyristor coupled to and supplying the induction heating coil with periodic energization currents, rectifier means for supplying a rectified unfiltered unidirectional high voltage excitation potential for said inverter circuit means and gating circuit mean for periodically gating on said inverter circuit means, said gating circuit means including means for deriving a low voltage unidirectional self excitation potential from the high voltage rectified unfiltered unidirectional excitation potential supplied from said rectifier means, timing circuit charging means supplied by said low voltage excitation potential, voltage responsive switching means connected to and controlled by said timing circuit charging means, said voltage responsive switching means being rendered conductive upon the voltage of said timing circuit charging means attaining a preset value, a semiconductor switching device having a relatively flat current vs voltage conducting characteristic and having its load terminals connected to the output from the rectifier means, the control gate of said constant current semiconductor switching device being connected to and supplied by the voltage responsive switching means and gating signal output circuit means connected in series circuit relationship with the constant current high voltage switching device and adapted for connection to the control gate of the controlled power thyristor comprising a part of inverter circuit means, selectively operable inhibit control means coupled to said gating circuit means for selectively inhibiting operation of said gating circuit means, and delay means for introducing a delay period intermediate selective operation of the inhibit means in a direction to remove the inhibit and enable operation of the gating circuit means.
 39. An improved induction heating unit power supply circuit and control according to claim 38 wherein said rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means.
 40. An improved induction heating unit power supply circuit and control according to claim 38 wherein the inverter circuit means is a chopper inverter comprised by a filter inductor having an inductance L2, said gate control semiconductor thyristor switching device being connected in series circuit relationship with the filter inductor across the output from said rectifier means with the filter inductor interposed between the thyristor switching device and the rectifier means, commutating inductor means having an inductance L1 and commutating capacitor means having a capacitance C1 connected in series circuit relationship across the thyristor switching device and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, said commutating capacitor and inductor means including as a part thereof the induction heating coil, the gating circuit means being coupled to the control gate of the thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period t2 in each cycle of operation where T t1 +t2 such that the value 2t2 equals substantially pi /2 radians or greater where omega 2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is maintained substantially independent of load.
 41. An improved induction heating unit power supply circuit and control according to claim 38 wherein the inverter circuit means comprises a chopper inverter circuit that produces periodic pulsed currents having radio frequency interference emission effects and the circuit further includes a smoothing inductor and a smoothing capacitor connected in circuit relationship across the chopper inverter circuit which serve to shape the output current supplied by the chopper inverter circuit to the load to substantially a sinusoidal wave shape having little or no radio frequency interference effects and the smoothing inductor comprises the induction heating coil.
 42. An improved induction heating unit power supply circuit and control according to claim 41 wherein the smoothing inductor has an inductance L3 and the smoothing capacitor has a capacitance C3 and are connected in series circuit relationship across at least one of the commutating components and have impedance values such that the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor is capacitive in nature and series resonates with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects.
 43. An improved induction heating unit power supply circuit and control according to claim 42 further including on-off control circuit means for turning said gating circuit means on and off in accordance with the power demands on the circuit, said on-off control circuit means controlling said inhibit control means for selectively clamping-off the excitation potential to the gating circuit means to thereby inhibit operation thereof, zero point sensing and turn-on circuit means coupled to and controlling said inhibit control means for enabling the gating circuit means only at or near the beginning of a half cycle of the rectified, unfiltered unidirectional excitation potential, operating condition sensing means for sensing the operating condition of the chopper inverter and deriving on-off control signal in response to the operating condition for controlling operation of the inhibit control means.
 44. An improved power supply circuit and control according to claim 43 wherein said rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of the supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means. 